Process for applying thermal barrier coatings to metals

ABSTRACT

Process for applying a protective coating to a metal substrate which provides a thermal barrier and a barrier against oxidation of the substrate. The coating material is a mixture of two metals M 1  and M 2 , e.g., cerium (M 1 ) and cobalt (M 2 ), one of which when exposed to an atmosphere containing a low partial pressure of oxygen and at a high temperature forms a stable oxide, the other of which does not form a stable oxide under such conditions. A coating consisting of such a metal alloy or mixture is subjected to such conditions to produce an outer oxide layer of metal M 1  and an inner metal layer of M 2  alloyed with one or more components of the substrate. The oxide layer provides thermal and oxidation protection and the inner layer bonds the coating to the substrate.

This invention relates to the coating of metals, particularly certainalloys, with a protective coating that acts as a thermal barrier.

Certain alloys known as "super alloys" are used as gas turbinecomponents where high temperature oxidation resistance and highmechanical strengths are required. In order to extend the usefultemperature range, the alloys must be provided with a coating which actsas a thermal barrier to insulate and protect the underlying alloy orsubstrate from high temperatures and oxidizing conditions to which theyare exposed.

Zirconium oxide is employed for this purpose because it has a thermalexpansion coefficient approximating that of the super alloys and becauseit functions as an efficient thermal barrier.

Zirconium oxide is applied to alloy substrates by plasma spraying, inwhich an inner layer or bond coat, for example NiCrAlY alloy, protectsthe superalloy substrate from oxidation and bonds to the superalloy andto the zirconium oxide. The zirconium oxide forms an outer layer orthermal barrier and the zirconia is partially stabilized with a secondoxide such a calcia, yttria or magnesia. The plasma spray techniquerequires two guns for application; it results in nonuniform coating; andit is not applicable or is difficultly applicable, to re-entrantsurfaces. The plasma sprayed coatings often have microcracks andpinholes that lead to catastrophic failure.

Thermal barrier coatings can also be applied using electron beamvaporization. This method of application is expensive and limited toline of sight application. Variations in coating compositions oftenoccur because of differences in vapor pressures of the coatingconstituent elements.

It is an object of the present investigation to provide improved methodsof applying thermal barrier coatings to metal substrates such as theaforesaid super alloys.

It is a particular object of the invention to provide an improvedprocess for applying such coatings to superalloys.

It is another object of the invention to provide structures comprising asubstrate of a metal, e.g. a super alloy or the like, having appliedthereto a thermal barrier coating in the form of a metal oxidesatisfying the requirements of thermal barriers and also resulting in auniform coating which is substantially free from cracks and otherdefects and is securely bonded to the substrate.

The above and other objects of the invention will be apparent from theensuing description and the appended claims.

In accordance with the present invention, an alloy or a physical mixtureof metals is provided comprising two metals M₁ and M₂ which are selectedin accordance with the criteria described below. This alloy or metalmixture is then melted to provide a uniform melt which is then appliedto a metal substrate by dipping the substrate in the melt.Alternatively, the metal mixture or alloy is reduced to a finely dividedstate, and the finely divided metal is incorporated in a volatilesolvent to form a slurry which is applied to the metal substrate byspraying or brushing. The resulting coating is heated to accomplishevaporation of the volatile solvent and the fusing of the alloy or metalmixture onto the surface of the substrate. (Where physical mixtures ofmetals are used, they are converted to an alloy by melting or they arealloyed in situ in the slurry method of application.)

The metals M₁ and M₂ are selected according to the following criteria:M₁ forms a thermally stable oxide when it is exposed to an atmospherecontaining a small concentration of oxygen such as that produced by amixture of carbon dioxide and carbon monoxide at a temperature of about900° C. The metal M₂, under such conditions, does not form a stableoxide and remains entirely or substantially entirely in the form of theunoxidized metal. Further, M₂ is compatible with the substrate alloy inthe sense that it extracts one or more of the components of thesubstrate to form an intermediate layer between the oxide outer layer(resulting from oxidation of M₁) and the substrate, such intermediatelayer being an alloy of M₁ and the extracted component or components andserving to bond the oxide layer to the substrate.

It will be understood that M₁ may be a mixture or alloy of two or moremetals meeting the requirements of M₁ and that M₂ may be a mixture oralloy of two or more metals meeting the requirements of M₂.

When a coating of suitable thickness has been applied to the substratealloy by the dip coating process or by the slurry process describedabove (and in the latter case after the solvent has been evaporated andthe M₁ /M₂ metal alloy or mixture is fused onto the surface of thesubstrate) the surface is then exposed to a selectively oxidizingatmosphere such as a mixture of carbon dioxide and carbon monoxide(hereinafter referred to as CO₂ /CO). A typical CO₂ /CO mixture contains90 percent of CO₂ and 10 percent of CO. When such a mixture is heated toa high temperature, an equilibrium mixture results in accordance withthe following equation:

    CO+1/2O.sub.2 =CO.sub.2

The concentration of oxygen in this equilibrium mixture is very small,e.g., at 800° C. the equilibrium oxygen partial pressure isapproximately 2×10⁻⁷ atmosphere, but is sufficient at such temperatureto bring about selective oxidation of M₁. Other oxidizing atmospheresmay be used, e.g., mixtures of oxygen and inert gases such as argon ormixtures of hydrogen and water vapor which provide oxygen partialpressures lower than the dissociation pressures of the oxides of theelements in M₂, and higher than the dissociation pressure of the oxideof M₁.

The coating thus formed and applied is then preferably subjected to anannealing step. The annealing step may be omitted when annealing occursunder conditions of use.

There results from this process a structure such as shown in FIG. 1 ofthe drawings.

Referring now to FIG. 1, this figure represents a cross-section througha substrate alloy indicated at 10 coated with a laminar coatingindicated at 11. The laminar coating 11 consists of an intermediatemetallic layer 12 and an outer oxide layer 13. The relative thicknessesof the layers 12 and 13 are exaggerated. The substrate layer 10 is asthick as required for the intended service.

The layers 12 and 13 together typically will be about 300 to 400micrometers thick, the layer 12 will be about 250 micrometers thick, andthe layer 13 will be about 150 micrometers thick. It will be understoodthat the layers 12 and 13 will have thicknesses adequate to form a firmbond with the substrate and to provide an adequate thermal and oxidationbarrier.

The metals M₁ and M₂ may, depending upon the type of service and thenature of the substrate alloy, be selected from Tables I and II,respectively.

                  TABLE I                                                         ______________________________________                                        (M.sub.1)                                                                     ______________________________________                                        Lanthanum      La       Holmium     Ho                                        Cerium         Ce       Erbium      Er                                        Praseodymium   Pr       Thulium     Tm                                        Neodymium      Nd       Ytterbium   Yb                                        Samarium       Sm       Lutetium    Lu                                        Europium       Eu       Actinium    Ac                                        Gadolinium     Gd       Thorium     Th                                        Terbium        Tb       Zirconium   Zr                                        Dysprosium     Dy       Hafnium     Hf                                        ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        (M.sub.2)                                                                     ______________________________________                                                Nickel         Ni                                                             Cobalt         Co                                                             Aluminum       Al                                                             Yttrium        Y                                                              Chromium       Cr                                                             Iron           Fe                                                     ______________________________________                                    

It will be understood that two or more metals chosen from Table I andtwo or more metals chosen from Table II may be employed to form thecoating alloy or mixture. Examples of suitable M₁ /M₂ metal mixtures are

                  TABLE III                                                       ______________________________________                                        M.sub.1                    M.sub.2                                            ______________________________________                                        Ce         +               Co                                                 Ce         +               Ni                                                 Ce         +               Co/Cr                                              Ce         +               Ni/Cr                                              Zr         +               Co                                                 Zr         +               Ni                                                 Sm         +               Co                                                 Sm/Ce      +               Co                                                 ______________________________________                                    

Proportions of M₁ and M₂ may vary from about 50 to 90% by weight of M₁to from about 10 to 50% by weight of M₂, preferably about 70 to 90% ofM₁ and about 10 to 30% of M₂. The proportion of M₁ should be sufficientto form an outer oxide layer sufficient to provide a thermal barrier andto inhibit oxidation of the substrate and the proportion of M₂ should besufficient to bond the coating to the substrate.

It will be noted that most of the metals in Table I are metals of thelanthanide series of elements. Such metals and zirconium are thepreferred choice for M₁.

Table IV provides examples of substrate alloys to which M₁ /M₂ areapplied in accordance with the present invention. It will be noted thatthe invention may be applied to superalloys in general and specificallyto cobalt and nickel based super alloys.

                  TABLE IV                                                        ______________________________________                                        Nickel Base Superalloy  IN 738                                                Cobalt Base Superalloy  MAR-M509                                              NiCrAlY Type Bond Coating Alloy                                               CoCrAlY Type Bond Coating Alloy                                               ______________________________________                                    

The invention may also be applied to any metal substrate which benefitsfrom a coating which is adherent and which provides a thermal barrierand/or protection from oxidation by the ambient atmosphere.

The dip coating method is preferred. In this method a molten M₁ /M₂alloy is provided and the substrate alloy is dipped into a body of thecoating alloy. The temperature of the alloy and the time during whichthe substrate is held in the molten alloy will control the thickness ofthe coating. The thickness of the applied coating can range between 100micrometers to 1000 micrometers. Preferably, a coating of about 300micrometers to 400 micrometers is applied. It will be understood thatthe thickness of the coating will be provided in accordance with therequirements of a particular end use.

The slurry fusion method has the advantage that it dilutes the coatingalloy or metal mixture and therefore makes it possible to effect bettercontrol over the thickness of coating applied to the substrate.Typically, the slurry coating technique may be applied as follows: Analloy of M₁ and M₂ is mixed with a mineral spirit and an organic cementsuch as Nicrobraz 500, (Well Colmonoy Corp.) and MPA-60 (Baker CoasterOil Co.). Typical portions used in the slurry are coating alloy 45weight percent, mineral spirit 10 weight percent, and organic cement, 45weight percent. This mixture is then ground, for example, in a ceramicball mill using aluminum oxide balls. After separation of the resultingslurry from the alumina balls, it is applied (keeping it stirred toinsure uniform dispersion of the particles of alloy in the liquidmedium) to the substrate surface and the solvent is evaporated, forexample, in air at ambient temperature or at a somewhat elevatedtemperature. The residue of alloy and cement is then fused onto thesurface by heating it to a suitable temperature, for example, 1250° C.in an inert atmosphere such as argon that has been passed over hotcalcium chips to getter oxygen. The cement will be decomposed and theproducts of decomposition are volatilized.

The following specific example will serve further to illustrate thepractice and advantages of the invention.

EXAMPLE 1

The substrate was a nickel base superalloy known as IN 738, which has acomposition as follows:

    ______________________________________                                        61%          Ni    1.75%          Mo                                          8.5%         Co     2.6%          W                                           16%          Cr    1.75%          Ta                                          3.4%         Al     0.9%          Nb                                          3-4%         Ti                                                               ______________________________________                                    

The coating alloy was in one case an alloy containing 90 percent ceriumand 10 percent cobalt, and in another case an alloy containing 90percent cerium and 10 percent nickel. The substrate was coated bydipping a bar of the substrate alloy into the molten coating alloy. Thetemperature of the coating alloy was 600° C., which is above theliquidus temperatures of the coating alloys. By experiment it wasdetermined that a dipping time of about one minute provided a coating ofsatisfactory thickness.

The bar was then extracted from the melt and was exposed to a CO₂ /COmixture containing 90.33 percent CO₂ and 9.67 percent CO. The exposureperiods ranged from 30 minutes to two hours and the temperature ofexposure was 800° C. The equilibrium oxygen partial pressure of the CO₂/CO mixture at 800° C. is 2.25×10⁻¹⁷ atmosphere, and at 900° C. it is7.19×10⁻¹⁵ atmosphere. The dissociation pressures of CoO were calculatedat 800° and 900° to be 2.75×10⁻¹⁶ atmosphere and 3.59×10⁻¹⁴ atmosphere,respectively, and the dissociation pressures of NiO were calculated tobe 9.97×10⁻¹⁵ atmosphere and 8.98×10⁻¹³ atmosphere respectively. Underthese circumstances neither cobalt nor nickel was oxidized.

Each coated specimen was then annealed in the absence of oxygen in ahorizontal tube furnace at 900° or 1000° C. for periods up to two hours.This resulted in recrystallization of oxide grains in the intermediatelayer.

Examination of the treated specimens, treated in this manner with thecerium cobalt alloy, revealed a structure in cross-section as shown inFIG. 2. In FIG. 2, as in FIG. 1, the thickness of the various layers isnot to scale, thickness of the layers of the coating being exaggerated.

Referring to FIG. 2, the substrate is shown at 10, an interaction zoneat 12A, a subscale zone at 12B and a dense oxide zone at 13. The denseoxide zone consists substantially entirely of CeO₂ ; the subscale zone12B contains both CeO₂ and metallic cobalt and the interaction zone 12Acontains cobalt and one or more metals extracted from the substrate.

Similar results are obtained using a cerium-nickel alloy containing 90%cerium and 10% nickel.

Such coatings provide thermal barriers suitable for such uses asdescribed above, they are adherent, and they do not undergo unacceptabledeterioration in use.

We claim:
 1. A method of coating a metal substrate with a protectivecoating which comprises:(a) providing a substrate metal to be coated,said substrate being a structural article suitable for use in amechanical structure having high mechanical strength, (b) providing analloy or mixture of at least one metal M₁, and at least one other metalM₂ selected according to the following criteria: (1) M₁ is susceptibleto oxidation by molecular oxygen at an elevated temperature in anatmosphere having a very small partial pressure of oxygen, suchoxidation resulting in a stable oxide of M₁, (2) M₂ does not form astable oxide under such conditions and it forms an alloy with at leastone component of the substrate on heat treatment of the coated material;(c) applying such alloy or mixture to a surface of the substrate, underconditions such that the surface only is coated with an alloy of M₁ andM₂ and (d) effecting selective oxidation of M₁ at an elevatedtemperature in the coating without substantial oxidation of M₂, (e) theproportion of M₁ to M₂ in said alloy or mixture of M₁ and M₂ beingsubstantial and sufficient to result in a coating containing sufficientoxide of M₁ to function as a substantial thermal barrier, (f) thequantity of M₁ and M₂ being adequate to form a firm bond with thesubstrate and to form a substantial thermal barrier.
 2. The method ofclaim 1 wherein after step (d) the coating is annealed.
 3. The method ofclaim 1 wherein the substrate metal is a superalloy.
 4. The method ofclaim 1 wherein M₁ is selected from the lanthanide metals.
 5. The methodof claim 4 wherein M₁ is cerium.
 6. The method of claim 1 wherein M₂ isselected from the group nickel, cobalt, aluminum, yttrium, chromium andiron.
 7. The method of claim 1 wherein the M₁ is cerium, M₂ is cobalt ornickel and the substrate metal is a superalloy.